The present invention is within the technical field of charge control agents in toners and developers for electrophotographic recording processes, in powders and powder coating materials for surface coating, in electret materials, especially in electret fibers, and in separation processes.
In electrophotographic recording processes a latent charge image is produced on a photoconductor. This latent charge image is developed by applying an electrostatically charged toner which is then transferred to, for example, paper, textiles, foils or plastic and is fixed by means, for example, of pressure, radiation, heat or the action of solvent. Typical toners are one- or two-component powder toners (also known as one- or two-component developers); also used are specialty toners, such as magnetic toners, liquid toners or polymerization toners, for example. By polymerization toners are meant those toners which are formed by, for example, suspension polymerization (condensation) or by emulsion polymerization and which lead to improved particle properties in the toner.
Also meant are those toners produced in principle in nonaqueous dispersions.
One measure of the quality of a toner is its specific charge q/m (charge per unit mass). In addition to the sign and level of the electrostatic charge, the principal, decisive quality criteria are the rapid attainment of the desired charge level and the constancy of this charge over an extended activation period. In addition to this, the insensitivity of the toner to climatic effects such as temperature and atmospheric humidity is a further important criterion for its suitability.
Both positively and negatively chargeable toners are used in copiers and laser printers, depending on the type of process and type of apparatus.
To obtain electrophotographic toners or developers having either a positive or negative charge, it is common to add charge control agents. Since the charge of toner binders is in general heavily dependent on the activation period, the function of a charge control agent is, on the one hand, to set the sign and level of the toner charge and, on the other hand, to counteract the charge drift of the toner binder and to provide for constancy of the toner charge.
Charge control agents which are not able to prevent the toner or developer from showing a high charge drift (aging) during a prolonged period of use, and which may even cause the toner or developer to undergo charge inversion, are hence unsuitable for practical use.
While for black toners it is possible to employ black, blue or dark charge control agents, coloristic factors demand, for color toners, charge control agents that have no inherent color.
In the case of full color toners, in addition to the precisely defined requirements in terms of color, the three toners, yellow, cyan and magenta, must also be matched exactly to one another in terms of their triboelectric properties, since they are transferred in succession in the same apparatus.
It is known that some colorants may have a sustained effect on the triboelectric charge of toners. Because of the different triboelectric effects of colorants and the resulting effect, sometimes very pronounced, on toner chargeability, it is not possible to simply add the colorants to a toner base formulation made available at the start. On the contrary, it may be necessary to make available for each colorant an individual formulation to which the nature and amount of the required charge control agent are specifically tailored.
Since this procedure is highly laborious, there is a need for highly effective, colorless charge control agents which are able to compensate for the different triboelectric characteristics of different colorants and to give the toner the desired charge. In this way, colorants which are very different triboelectrically can be employed in the various toners required (yellow, cyan, magenta and if desired black) using one and the same charge control agent, on the basis of a toner base formulation made available at the start.
Another important practical requirement is that the charge control agents should have high thermal stability and good dispersibility. Typical temperatures at which charge control agents are incorporated into the toner resins, when using kneading apparatus or extruders, are between 100.degree. C. and 200.degree. C. Correspondingly, thermal stability at 200.degree. C. is a great advantage. It is also important for the thermal stability to be ensured over a relatively long period (about 30 minutes) and in a variety of binder systems. This is significant because matrix effects occur again and again and lead to the premature decomposition of the charge control agent in the toner resin, causing the toner resin to turn dark yellow or dark brown and the charge control effect to be wholly or partly lost. Typical toner binders are addition polymerization, polyaddition and polycondensation resins, such as styrene, styrene-acrylate, styrene-butadiene, acrylate, polyester and phenol-epoxy resins, and also cycloolefin copolymers, individually or in combination, which may also include further components, examples being colorants, such as dyes and pigments, waxes or flow assistants, or may have these components added subsequently, such as highly disperse silicas.
For good dispersibility it is of great advantage if the charge control agent has minimal waxlike properties, no tackiness, and a melting or softening point of &gt;150.degree. C., more preferably &gt;200.degree. C. Tackiness leads frequently to problems in the course of the metered addition of the charge control agent to the toner formulation, and low melting or softening points may result in a failure to attain homogeneous distribution in the course of dispersing, since the material coalesces in droplets in the carrier material.
Apart from their use in electrophotographic toners and developers, charge control agents may also be used to improve the electrostatic charge of powders and coating materials, especially in triboelectrically or electrokinetically sprayed powder coatings as are used to coat surfaces of articles made from, for example, metal, wood, plastic, glass, ceramic, concrete, textile material, paper or rubber. Power coating technology is used, for example, when coating articles such as garden furniture, camping equipment, domestic appliances, vehicle parts, refrigerators and shelving and for coating workpieces of complex shape. The powder coating material, or the powder, receives its electrostatic charge, in general, by one of the two following processes:
In the corona process, the powder coating material or the powder is guided past a charged corona and is charged in the process; in the triboelectric or electrokinetic process, the principle of frictional electricity is utilized.
The powder coating material or the powder in the spray apparatus receives an electrostatic charge which is opposite to the charge of its friction partner, generally a hose or spray line made, for example, from polytetrafluoroethylene.
It is also possible to combine the two processes. Typical powder coating resins employed are epoxy resins, carboxyl- and hydroxyl-containing polyester resins, polyurethane resins and acrylic resins, together with the customary hardeners. Resin combinations are also used. For example, epoxy resins are frequently employed in combination with carboxyl- and hydroxyl-containing polyester resins.
Examples of typical hardener components for epoxy resins are acid anhydrides, imidazoles and dicyandiamide, and derivatives thereof. Examples of typical hardener components for hydroxyl-containing polyester resins are acid anhydrides, blocked isocyanates, bisacylurethanes, phenolic resins and melamine resins. For carboxyl-containing polyester resins, typical hardener components are, for example, triglycidyl isocyanurates or epoxy resins. Typical hardener components used in acrylic resins are, for example, oxazolines, isocyanates, triglycidyl isocyanurates or dicarboxylic acids.
The disadvantage of insufficient charging can be seen above all in triboelectrically or electrokinetically sprayed powders and powder coating materials which have been prepared using polyester resins, especially carboxyl-containing polyesters, or using so-called mixed powders, also referred to hybrid powders. By mixed powders are meant powder coating materials whose resin base comprises a combination of epoxy resin and carboxyl-containing polyester resin. The mixed powders form the basis for the powder coating materials used most commonly in practice. Inadequate charging of the abovementioned powders and powder coating materials results in an inadequate deposition rate and inadequate throwing power on the workpiece to be coated. The term "throwing power" is a measure of the extent to which a powder or powder coating material is deposited on the workpiece to be coated, including its rear faces, cavities, fissures and, in particular, its inner edges and corners.
It has additionally been found that charge control agents are able to improve considerably the charging and the charge stability properties of electret materials, especially electret fibers (DE-A-43 21 289). Electret fibers have hitherto been described mainly in connection with the problem of filtering very fine dusts. The filter materials described differ both in respect of the materials of which the fibers consist and with regard to the manner in which the electrostatic charge is applied to the fibers. Typical electret materials are based on polyolefins, halogenated polyolefins, polyacrylates, polyacrylonitriles, polystyrenes or fluoropolymers, for example polyethylene, polypropylene, polytetrafluoroethylene and perfluorinated ethylene and propylene, or on polyesters, polycarbonates, polyamides, polyimides, polyether ketones, on polyarylene sulfides, especially polyphenylene sulfides, on polyacetals, cellulose esters, polyalkylene terephthalates and mixtures thereof. Electret materials, especially electret fibers, can be used, for example, to filter (very fine) dusts. The electret materials can receive their charge in a variety of ways, for instance by corona or triboelectric charging.
It is additionally known that charge control agents can be used in electrostatic separation processes, especially in processes for the separation of polymers. For instance, using the example of the externally applied charge control agent trimethylphenylammonium tetraphenylborate, Y. Higashiyama et al. (J. Electrostatics 30 (1993), pp. 203-212) describe how polymers can be separated from one another for recycling purposes. Without charge control agents, the triboelectric charging characteristics of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are extremely similar. Following the addition of charge control agent, LDPE takes on a highly positive and HDPE a highly negative charge, and the materials can thus be separated easily. In addition to the external application of the charge control agents it is also possible to conceive in principle of their incorporation into the polymer in order, for example, to shift the position of the polymer within the triboelectric voltage series and to obtain a corresponding separation effect. In this way it is likewise possible to separate other polymers, such as polypropylene (PP) and/or polyethylene terephthalate (PET) and/or polyvinyl chloride (PVC), from one another.
Salt minerals, for example, can likewise be separated with particularly good selectivity if they are surface-treated beforehand (surface conditioning) with an additive which improves the substrate-specific electrostatic charging (A. Singewald, L. Ernst, Zeitschrift fur Physikal. Chem., Neue Folge, Vol. 124, (1981) pp. 223-248).
Charge control agents are employed, furthermore, as electroconductivity providing agents (ECPAs) for inks in inkjet printers (JP 05 163 449-A).
Charge control agents are known from numerous literature references. However, the charge control agents known to date have a number of disadvantages, which severely limit their use in practice or even, in some cases, render it impossible; examples of such disadvantages are inherent color, instability to heat or light, low stability in the toner binder, inadequate activity in terms of the desired sign of the charge (positive or negative charging), charge level or charge constancy, and dispersibility.